As fiber optic communication systems continue to push to even higher data rates and larger bandwidths, the availability of reproducible and reliable high-speed lasers becomes increasingly important. This is particularly true for microwave modulation schemes like subcarrier multiplexing, where low relative intensity noise (RIN) is very important, or for phase modulated systems, where the avoidance of phase noise is crucial. In both of these modulation schemes it is very important to avoid operating near the resonance peak of the laser, where RIN peaks and a .pi. phase shift occurs. Both of these phenomena have a deleterious effect on the noise performance of the system. The further below the resonance peak the fiber optic system can be designed to operate, the better its noise performance will be. Clearly, demands on the system designer are reduced by the availability of lasers with higher resonance frequencies and modulation bandwidths.
Fabrication of single transverse mode semiconductor lasers with modulation bandwidth in excess of 15 GHz depends heavily upon both the accurate control of the active layer doping, width, and thickness, and upon providing a lateral optical cladding of the active layer which minimizes surface recombination and carrier leakage while not affecting the single transverse modal properties of the laser. In addition, a low capacitance and low series resistance structure is required to minimize electrical parasitics.
High frequency single transverse mode semiconductor lasers have been achieved by constricted mesa (Bowers, et al, "High-speed InGaAsP constricted mesa lasers", IEEE J. Quantum Electron., Vol. QE-22, pp. 833-884, June 1986), mass transport (Liau, et al, "A novel technique for GaInAsP/InP buried heterostructure fabrication", Appl. Phys. Lett., Vol. 40, pp. 568-570, April 1982) and vapor phase regrowth (Su, et al, "Ultra-high frequency modulation of InGaAsP lasers", Tech. Dig. Conf. Optical Fiber Communication, pp. 90-91, February 1986) techniques. These structures have broad mesa tops so as to reduce series resistance and facilitate the formation of ohmic contacts. In order to achieve this structure, the processes reported to date require selective wet chemical etching of the active layer of 4 microns or more to reduce its width and/or the epitaxial growth of thick layers of high quality p-doped, semi-insulating, or pn blocking layer configurations. These regrown materials are subject to unintentional doping to and from the p-type device structure layers during the regrowth process and tend to be electrically leaky which severely limits the achievable output power and bandwidth. (Ohtoshi, et al, "Current leakage mechanism in InGaAsP/InP buried heterostructure lasers", 11th IEEE International Semiconductor Laser Conference, Boston, Mass., 1988). In addition, wet chemical etching through dielectric masks, used to form the initial mesa structure, is inherently non-uniform so that control of active layer width, a necessity for high bandwidth single transverse mode operation, is very difficult resulting in low yield and non-reproducible results. Alternatively, the fabrication of narrow mesa tops leads to the additional problems of high series resistance and difficulty in photolithographically defining and forming ohmic contacts.